1. FIELD OF THE INVENTION
This invention relates to a rotor of a permanent magnet type synchronous motor and more particularly to a rotor structure of a permanent magnet type synchronous motor which is suitable for a high speed rotation.
1. DESCRIPTION OF THE PRIOR ART
Widespread use is being made to a permanent magnet type synchronous motor having a rotor structure incorporated with permanent magnets and squirrel-cage type windings since the synchronous motor of this type is easy to operate, of high efficiency and high power factor. Particular application is advantageously made to textile machines such as spinning machines since when operated in parallel, a number of synchronous motors of this type can easily be uniformed for their speed controlling.
The permanent magnet type synchronous motor has a rotor structure incorporated with permanent magnets which undergo a large centrifugal force during its rotation and hence it requires a mechanically strong rotor. Especially, for a super high speed rotation of about 10,000 r.p.m., it is necessary to provide the rotor with a construction resistive to an accordingly large centrifugal force.
Referring to FIGS. 1 to 4, there is schematically shown a principal part of a permanent magnet type synchronous motor, as a prior art, which comprises a rotor structure having an reinforcement plate interposed between rotor cores for the purpose of promoting the resistivity to the large centrifugal force.
As will be seen from these figures, a rotor structure R comprises permanent magnets 1 and 1' made of ferrite, a rotary shaft 2 made of a non-magnetic material, rotor cores 3 each of which is in the form of a lamination of silicon steel sheets which is stacked in the axial direction, non-magnetic reinforcement plates 4 each of which is in the form of a lamination of stainless steel sheets which is stacked in the axial direction, end plates 5 mounted on both ends of the rotor, and end rings for short-circuiting squirrel-cage windings at their ends. A stator structure S opposing the rotor structure comprises a stator core 11 and armature windings 12 wound on the stator core.
Sector cavities, the number of which corresponds to that of magnet poles such that for a two-magnet pole motor, for example, a pair of cavities are provided, are formed axially and symetrically with the rotary shaft in the rotor core and reinforcement laminations by passing therethrough, and as shown in FIG. 2 the permanent magnets 1 and 1' are received in the sector cavities. Thus, the permanent magnet is protected against vibrations and shocks by the rotor core and the reinforcement plate. To prevent the short-circuit of magnetic flux from the permanent magnets 1 and 1', the rotor core 3 is formed with respective slits 8 each of which is located between one end of the permanent magnet 1 and one end of the permanent magnet 1', bridges 9 connecting inner and outer portions of the rotor core have each a narrow width and in addition, relatively large air gaps 10 are formed between sides of bridges 9 and sides of permanent magnets 1 and 1'. The slit 8, narrow width bridge 9 and air gap 10, however, impair the mechanical strength of the rotor core 3.
On the other hand, the reinforcement plate 4 made of a non-magnetic material such as stainless steel sheet has a configuration as shown in FIG. 3. As being made of the non-magnetic material and posing no problem of the magnetic flux sheet circuit, the reinforcement plate is dispensed with the slit 8 and it has bridges 9' of a large width, making it possible to make small air gaps 10' between sides of the permanent magnets 1 and 1' and sides of the bridges 9'. In some cases, when diecasting aluminum to form the squirrel-cage windings 7, the air gaps are also filled with die-casting aluminum. By virture of this construction, the reinforcement plate 4 as shown in FIG. 3 is mechanically robust.
In other words, a plurality of the non-magnetic reinforcement plates 4 disposed axially can afford a robust rotor structure of permanent magnet type synchronous motor operating at a high speed.
However, the reinforcement plate 4 and rotor core 3 are made of different materials which differ in hardness, temperature coefficient and the like properties and hence they undergo different thermal expansions due to variation in temperature during manufacture, resulting in difficulties for making identical in dimension the reinforcement plate and rotor core. In operation, the reinforcement plate and the rotor core also undergo different thermal expansions due to temperature rise with the result that the radial dimension of the cavity becomes different one portion corresponding to the rotor core to the other portion corresponding to the reinforcement plate. This imposes an excessive local load on the permanent magnets 1 and 1'.
This problem will be detailed with reference to FIG. 4 illustrating the principal part of rotor structure. As shown therein, the outer surface of the permanent magnet 1 is in intimate contact with the reinforcement plate 4 by a large centrifugal force during rotation of the rotor whereas a small air gap g.sub.1 lies between this outer surface and the inner surface of the cavity in the rotor core 3. Between the inner surface of the permanent magnet 1 and the reinforcement plate 4 lies a small air gap g.sub.2.
Under these conditions during rotation of the rotor at a high speed, local cracks k take place at stressed points on the permanent magnet 1 mainly near the boundary between the rotor core 3 and the reinforcement plate 4. As these cracks k grow and increase, the permanent permanent magnet 1 tends to fracture into segments, giving rise to cause for rotational unbalance of the rotor. In consequence, unwanted vibrations added to the normal rotary movement of the rotor increase and hence bearings on which the drive shaft is journaled are loaded excessively.